Stabilization of low molecular weight polybutylene terephthalate/polyester blends with phosphorus compounds

ABSTRACT

Blends of low molecular weight PBT resin and high molecular weight polyester resin, such as high molecular weight PBT resin, are imparted with excellent melt viscosity stability by the addition of certain phosphorus-containing compounds.

This is a divisional of application Ser. No. 08/164,642, filed Dec. 8,1993, now U.S. Pat. No. 5,367,011 which is a continuation of Ser. No.07/994,738, filed Dec. 22, 1992, now abandoned.

BACKGROUND OF THE PRESENT INVENTION

Polyester resins derived from terephthalic acid and reactive derivativesthereof, such as dimethylene terephthalate, and alkane diols, e.g., offrom 1 to 10 carbon atoms, e.g., ethylene glycol, and 1,4-butanediol, aswell as related diols, such as 1,4-cyclohexane dimethanol, and mixturesof such resins have been known for some time and have become importantconstituents in injection moldable compositions. Workpieces molded fromsuch polyester compositions, alone, or combined with reinforcments,offer a high degree of surface hardness and abrasion resistance, highgloss, and lower surface friction.

Accordingly, polyesters have found significant commercial applications.Polyesters are effectively used as an engineering plastic for electricalcomponents, machines, cars, sporting goods, interior decorative goodsand the like. For many of these applications it is desirable to employ apolyester resin having a relatively high molecular weight, i.e., havinga melt viscosity of above about 600 poise as measured using a TiniumOlsen melt indexer at 250° C., 0.042 inch orifice (ASTM method D-1238);or having an intrinsic viscosity of above about 0.6 decaliters/gram asmeasured using a 120-130 mg sample of polyester in a 3:2 mixture ofphenol/tetrachloroethane and measuring the time of flow with a Ubbelohdecapillary viscometer at 25° C.

However, a problem which many plastics operators have exprienced withhigh molecular weight polyesters is the difficulty in processing theresins. In response thereto, a relatively low molecular weight PBT, lessthan about 600 poise, as measured using a Tinius Olsen melt indexer at250° C., 0.042 inch orifice (ASTM method D-1238) has been added to thehigh molecular weight polyester to provide improved flow blends whichstill retain the excellent properties of the high molecular weightpolyester. However, it has been found that while the blends of highmolecular weight polyester and low molecular weight PBT initiallyexhibit a decrease in melt viscosity, the melt viscosity builds overtime. See FIG. 1. Such a build up of viscosity of the blends limits theuse of low molecular weight PBT in commercial applications. It wouldtherefore represent a notable advance in the state of the art if a morestable high flow blend of a high molecular weight polyester and lowmolecular weight PBT could be found.

Jacquiss et al., U.S. Pat. No. 4,532,290, teach stabilizingpolycarbonate-polyester compositions against undesirable changes inmelting point by adding monosodium phosphate and/or monopotassiumphosphate to the compositions. Hepp, U.S. Pat. No. 4,687,802, disclosesthat the arc track rate of PBT resins can be improved by the addition ofa metal salt to the PBT resin.

However, none of the prior art teachings suggests a method of preparinga stable high flow blend of a high molecular weight polyester and a lowmolecular weight PBT. Surprisingly, the present inventor has now foundthat if certain phosphorus-containing compounds are added to the blendsof high molecular weight polyester and low molecular weight PBT, thereis achieved a high flow blend having excellent melt stability.

BRIEF DESCRIPTION OF THE DRAWNINGS

FIG. 1 depicts in graphic form a comparison of a time sweep of the meltviscosity of an unstabilized blend of a high molecular weight PBT (1100poise) resin and a low molecular weight (300 poise) PBT resin (Line A)against a time sweep of a high molecular weight PBT resin (Line B).

FIG. 2 depicts in graphic form a comparison of time sweeps of blends ofhigh molecular weight PBT and low molecular weight PBT with variousadditives from the data set forth in Table 1 hereinbelow.

SUMMARY OF THE PRESENT INVENTION

According to the present invention there is disclosed a thermoplasticresin blend comprising: (a) a relatively low molecular weightpolybutylene terephthalate resin; (b) a relatively high molecular weightpolyester resin; and (c) an effective melt viscosity stabilizing amountof (i) an acidic phosphate salt, (ii) an acid, alkyl, aryl or mixedphosphite having at least one hydrogen or alkyl group, (iii) a Group IBor IIB metal phosphate salt (iv) a phosphorous oxo acid or (v) a mixtureof any of the foregoing.

Also according to the present invention there is provided a process forstabilizing the melt viscosity of a thermoplastic resin blend comprisinga relatively low molecular weight PBT resin and a relatively highmolecular weight polyester resin comprising adding to the blend aneffective amount of a (i) an acidic phosphate salt, (ii) an acid, alkyl,aryl or mixed phosphite having at least one hydrogen or alkyl group,(iii) a Group IB or IIB metal phosphate salt (iv) a phosphorous oxo acidor (v) a mixture of any of the foregoing.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The low molecular weight polybutylene terephthalate (PBT) useful in thepractice of the present invention is a PBT resin having a melt viscosityof less than 600 poise, more preferably less than about 450 poise, andmost preferably less than about 300 poise.

The high molecular weight polyester resins useful in the practice of thepresent invention are polyester resins having a melt viscosity aboveabout 600 poise, more preferably above about 900 poise, and mostpreferably above about 1100 poise. Preferred as the high molecularweight polyester resins are high molecular weight polybutyleneterephthalate, polyethylene terephthalate and polycyclohexanedimethylene terephthalate.

The low molecular weight polybutylene terephthalate resin employed inthe present invention is typically one obtained by polymerizing a glycolcomponent at least 70 mol %, preferably at least 80 mol %, of whichcomprises a tetramethylene glycol; and an acid component at least 70 mol%, preferably at least 80 mol %, of which comprises terephthalic acid,and polyester-forming derivatives thereof. Particularly useful ispoly(1,4-butylene terephthalate).

Preferably, the glycol does not contain more than 30 mol %, morepreferably not more than 20 mol %, of another glycol, such as ethyleneglycol, trimethylene glycol, 2-methyl-1,3-propane glycol, hexamethyleneglycol, decamethylene glycol, cyclohexane dimethanol, or neopentyleneglycol. Examples of other copolycondensable polyols include1,3-propylene glycol, pentaerythritol, 1,6-hexanediol, polyethyleneglycol and polytetramethylene glycol.

Preferably the acid component contains not more than 30 mol %, morepreferably not more than 20 mol %, of another acid such as isophthalicacid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylicacid, 1,5-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid,4,4'-diphenoxyethanedicarboxylic acid, p-hydroxy benzoic acid, sebacicacid, adipic acid and polyester-forming derivatives thereof. Examples ofother copolycondensable polycarboxylic acids include azelaic acid,dodecane dicarboxylic acid, trimellitic acid, trimesic acid andhexahydroterephthalic acid.

The low molecular weight PBT resins can be prepared according to methodsknown to those of ordinary skill in the art, or they may be obtainedcommercially. Low molecular weight PBT is VALOX® 195 having a meltviscosity of about 300 poise from General Electric Company.

The high molecular weight polyester resins are those which are preparedin similar manner to the low molecular weight PBT, i.e., by polymerizinga glycol component and an acid component. Typically they are derivedfrom an aliphatic or cycloaliphatic diol, or mixtures thereof,containing from 2 to about 10 carbon atoms and at least one aromaticdicarboxylic acid. Preferred polyesters are derived from an aliphaticdiol and an aromatic dicarboxylic acid and have repeating units of thefollowing general formula: ##STR1## wherein n is an integer of from 2 to6. The most preferred polyesters are poly(ethylene terephthalate), poly(1,4-butylene terephthalate) and mixtures thereof.

Also contemplated for use herein as the high molecular weightpolyesters, are the above polyesters with minor amounts, e.g., from 0.5to about 5 percent by weight, of units derived from aliphatic acidsand/or aliphatic polyols to form copolyesters. The aliphatic polyolsinclude glycols, such as poly(ethylene glycol). All such polyesters canbe made following the teachings of, for example, U.S. Pat. Nos.2,465,319 and 3,047,539.

Also useful as the high molecular weight polyesters are those derivedfrom a cycloaliphatic diol and an aromatic dicarboxylic acid and whichare prepared, for example, by condensing either the cis- or trans-isomer (or mixtures thereof) of, for example, 1,4-cyclohexanedimethanolwith an aromatic dicarboxylic acid so as to produce a polyester havingrecurring units of the following formula: ##STR2## wherein thecyclohexane ring is selected from the cis- and trans- isomers thereofand R represents an aryl radical containing from 6 to about 20 carbonatoms and which is the decarboxylated residue derived from an aromaticdicarboxylic acid.

Examples of aromatic dicarboxylic acids represented by thedecarboxylated residue R are isophthalic or terephthalic acid,1,2-di-(p-carboxyphenyl)ethane, 4,4'-dicarboxydiphenyl ether, etc., andmixtures of these. All of these acids contain at least one aromaticnucleus. Acids containing fused rings can also be present, such as in1,4- or 1,5-naphthalenedicarboxylic acids. The preferred dicarboxylicacids are terephthalic acid or a mixture of terephthalic and isophthalicacids.

Another useful high molecular weight polyester may be derived from thereaction of either the cis- or trans- isomer (or a mixture thereof) of1,4-cyclohexanedimethanol with a mixture of isophthalic and terephthalicacids. Such a polyester would have repeating units of the formula:##STR3##

Still another useful high molecular weight polyester is a copolyesterderived from a cyclohexanedimethanol, an alkylene glycol and an aromaticdicarboxylic acid. These copolyesters are prepared by condensing eitherthe cis- or trans- isomer (or mixture thereof) of, for example,1,4-cyclohexanedimethanol and an alkylene glycol with an aromaticdicarboxylic acid so as to produce a copolyester having units of theformulae: ##STR4## wherein the cyclohexane ring is selected from thecis- and trans- isomers thereof, R is as previously defined, n is aninteger of 2 to 6, the x units comprise from about 10 to about 90percent by weight and the y units comprise from about 90 to about 10percent by weight.

Such copolyesters may be derived from the reaction of either the cis- ortrans- isomer (or mixtures thereof) of 1,4-cyclohexanedimethanol andethylene glycol with terephthalic acid in a molar ratio of 1:2:3. Thesecopolyesters have repeating units of the following formulae: ##STR5##wherein x and y are as previously defined.

The high molecular weight polyesters described herein are eithercommercially available or can be produced by following methods wellknown in the art, such as those set forth in, for example, U.S. Pat.Nos. 2,901,466, 2,465,319 and 3,047,539.

Particularly suitable for practice of the present invention is highmolecular weight PBT, which is sold commercially as VALOX® 295 (meltviscosity of about 1100 poise) or VALOX® 315 (melt viscosity of about8500 poise), both available from General Electric Company.

The PBT blend can comprise the low molecular weight PBT resin in amountsranging from about 5 to about 95, preferably from about 20 to about 80,more preferably from about 30 to about 70 and most preferably from about40 to about 60 parts by weight based on 100 total parts by weight of thelow molecular weight PBT and high molecular weight polyester resinstaken together. Accordingly, the PBT blend comprises the high molecularweight polyester resin in amounts ranging from about 95 to about 5,preferably from about 90 to about 10, more preferably from about 70 toabout 30 and most preferably from about 60 to about 40, parts by weightbased on 100 total parts by weight of the low molecular weight PBT andhigh molecular weight polyester resins taken together.

The phosphorus-containing stabilizers of the present invention maycomprise (i) acidic phosphate salts such as monozinc phosphates, sodiumdihydrogen phosphate, potassium hydrogen phosphate, calcium hydrogenphosphate, sodium acid pyrophosphate and mixtures thereof. It has alsobeen found that certain phosphite compounds (ii) may also be used in thepractice of the present invention, e.g., those of the general formulaP-(OR')₃ wherein each R' is the same or different and independentlyrepresents hydrogen, alkyl groups, aryl groups or any mixture thereofprovided that at least one of the R' groups is hydrogen or alkyl.Illustratively, these include, but are not limited to, diphenylisodecylphosphite, diisooctyl phosphite, dilauryl phosphite, diphenyl phosphite,phenyl diisodecyl phosphite, ethyl hexyl diphenyl phosphite, stearylphosphite and mixtures thereof. The phosphorus-containing stabilizersmay also comprise (iii) Group IB or Group IIB phosphate salts such aszinc phosphate or (iv) phosphorous oxo acids such as phosphorous acid,phosphoric acid, polyphosphoric acid, or hypophosphorous acid.

Preferred are phosphorus-containing compounds selected from zincphosphate, diphenylisodecyl phosphite, monosodium phosphate and sodiumacid pyrophosphate and mixtures thereof. Most preferred is zincphosphate.

The phosphorus-containing compounds are generally employed in thecompositions of the present invention in amounts ranging from about 0.1to about 10, preferably from about 0.1 to about 5, more preferably fromabout 0.1 to about 2 and most preferably from about 0.2 to about 1,weight percent based on the weight of the total composition.

The compositions of the present invention may also comprise otherthermoplastic resins which are conventionally added to polyester resins.These may include resins such as polycarbonates, polyestercarbonates,polyarylates and mixtures thereof.

In other embodiments of the present invention, the compositions canfurther comprise impact modifiers. Particularly useful impact modifiersgenerally comprise rubbery impact modifiers. These are well known tothose skilled in the art, and any of them normally employed withpolyester resins may be employed herein.

The preferred impact modifiers generally comprise an acrylic ormethacrylic grafted polymer of a conjugated diene or an acrylateelastomer, alone, or copolymerized with a vinyl aromatic compound.Particularly useful are the core-shell polymers of the type availablefrom Rohm & Haas, for example, those sold under the trade designationAcryloid®. In general these impact modifiers contain units derived frombutadiene or isoprene, alone or in combination with a vinyl aromaticcompound, or butyl acrylate, alone or in combination with a vinylaromatic compound. The aforementioned impact modifiers are believed tobe disclosed in Fromuth et al., U.S. Pat. Nos. 4,180,494; Owens,3,808,180; Farnham et al., 4,096,202; and Cohen et al., 4,260,693. Mostpreferably, the impact modifier will comprise a two stage polymer havingeither a butadiene or butyl acrylate based rubbery core and a secondstage polymerized from methylmethacrylate alone, or in combination withstyrene. Also present in the first stage are crosslinking and/orgraftlinking monomers. Examples of the crosslinking monomers include1,3-butylene diacrylate, divinyl benzene and butylene dimethacrylate.Examples of graftlinking monomers are allyl acrylate, allyl methacrylateand diallyl maleate.

Additional preferred impact modifiers are of the type disclosed in U.S.Pat. No. 4,292,233. These impact modifiers comprise, generally, arelatively high content of a cross-linked butadiene polymer grafted basehaving grafted thereon acrylonitrile and styrene.

Other suitable impact modifiers include, but are not limited to ethylenevinyl acetate, ethylene ethylacrylate copolymers, SEBS (styreneethylene-butylene styrene) and SBS (styrene-butadiene-styrene) blockcopolymers, EPDM (ethylene propylene diene monomer) and EPR (ethylenepropylene rubber) copolymers, etc. All of these are well known to thoseskilled in the art and are available commercially.

The compositions of the present invention may further contain one ormore reinforcing agents including glass fibers. Typical reinforcingagents useful in the practice of the present invention include, but arenot limited to, glass fiber, talc, mica, clay or combinations thereof.

The filamentous glass which may be employed as a reinforcing agent inthe present compositions is well known to those skilled in the art andis widely availiable from a number of manufacturers. For compositions tobe employed for electrical uses, it is preferred to use fibrous glassfilaments comprised of lime-aluminum borosilicate glass that isrelatively soda-free. This is more commonly known as "E" glass. However,other glasses are useful where electrical propeties are not soimportant, e.g., the low soda glass known as "C" glass. The filamentsare made by standard processes, e.g., by steam or air blowing, flameblowing and mechanical pulling. The filament diameters generally rangefrom about 0.00012 to 0.00075 inches but this is not critical to thepresent invention.

Further, the glass fibers useful in the practice of the presentinvention may also be treated with functionalized silicon compounds toimprove interaction with the polymer matrix, as is well known to thoseskilled in the art. Functionalized silanes, especially alkoxy silanesmay be useful in this regard. Illustratively these include, but are notlimited to, aminopropyl triethoxy silane, glycidyl propyl trimethoxysilane, (3,4-epoxy cyclohexyl) ethyl triethoxy silane, mercaptopropylsilane, aminoethyl aminopropyl alkoxy silane, ureido-alkyl trialkoxysilane and mixtures of any of the foregoing.

The length of the glass filaments and whether or not they are bundledinto fibers and the fibers bundled in turn to yarns, ropes or rovings,or woven into mats and the like are also not critical to the presentinvention. However, in preparing molding compositions it is convenientto use the filamentous glass in the form of chopped strands of fromabout 0.0125 to about 2 inches long. In articles molded from thecompositions on the other hand, even shorter lengths will be encountereddue to fragmentation during compounding.

In general, the filamentous glass reinforcement comprises from about 2.5to about 60% by weight based on the total weight of the compositions ofthe present invention. It is more preferred that the glass comprise fromabout 5 to about 55 and most preferred from about 20 to about 40% byweight of the total weight of the composition.

The present invention may further comprise a flame retardant compound.Any of the conventional halogenated aromatic flame retardants such asdecabromodiphenyl ether, brominated phthalimides, brominatedpolyphenylene ethers, bromine containing polyacrylates or methacrylates,i.e., polypentabromobenzyl acrylate and/or brominated styrene polymerscan be employed in the present invention. These are well known to thoseskilled in the art and are described in the patent literature. Preferredare derivatives of tetrabromo bisphenol A, such as its polycarbonatepolymer or the polymer of its adduct with epichlorohydrin (brominatedphenoxy resin). They may be used alone, or in conjunction with asynergist, particularly inorganic or organic antimony compounds. Suchcompounds are widely available or can be made in known ways. Especiallypreferred is antimony oxide.

Flame retardant embodiments of the present invention may furthercomprise a drip retardant agent to prevent dripping during burning. Suchcompounds are well known to those skilled in the art and include, butare not limited to, various fluorinated polyolefins. Particularly usefulis polytetrafluoroethylene (PTFE). See, e.g., Wambach, U.S. Pat. No.3,671,487.

The compositions of the present invention can also comprise a widevariety of other additives, such as UV stabilizers, pigments, colorants,fillers, plasticizers, processing aids, antioxidants and the like. Suchcomponents are added in effective amounts to impart the desiredproperties on the compositions of the present invention for the specificapplication.

The method of blending the compositions of the present invention is notcritical and can be carried out by conventional melt processingtechniques. One convenient method comprises blending the PBT resins andother ingredients in powder or granular form, extruding the blend andcomminuting into pellets or other suitable shapes. The ingredients arecombined in any usual manner, e.g., by dry mixing or by mixing in themelted state in an extruder, on a heated mill or in other mixers.

An alternative method of blending can comprise preparing a preblend ofthe polyesters and then adding the other ingredients to the preblend.For example, a preblend of the PBT resins and stabilizer can be fed intothe upstream port of an extruder with addition of the other ingredientssuch as glass fibers in a downstream port of the extruder.

In another embodiment, the various compounds can be precompounded,pelletized and then molded. Precompounding can be carried out inconventional equipment. For example, a dry blend of the ingredients canbe fed into a single screw extruder, the screw having a long transitionsection to insure proper melting. Alternatively, a twin screw extrusionmachine can be fed with the resins and other additives at the feed portand reinforcements fed downstream. In either case, a generally suitablemachine temperature will be from about 450° to about 575° F.

The precompounded composition can be extruded and cut or chopped intomolding compounds, such as conventional granules, pellets, etc. bystandard techniques.

The compositions can be molded in any equipment conventionally used forthermoplastic compositions. For example, good results will be obtainedin an injection molding machine, with conventional cylindertemperatures, e.g., 500° F., and conventional mold temperatures, e.g.,150° F.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples are presented in order to illustrate the presentinvention. They are not to be construed to limit the scope of theappended claims in any manner whatsoever.

EXAMPLES 1-3

The following examples demonstrate the use of various stabilizers in apolyester resin blend. The blends are prepared by tumble blending theingredients (parts by weight), extruding in a 2.5 inch vented singlescrew extruder with a melt temperature of 500 to 530° F. They are driedand molded on an 80 ton Van Dorn injection molding machine at atemperature of 500° C., and a mold temperature of 150° C. Kayness® meltviscosity is measured by predrying the sample for 1 hour in acirculating oven at 150° C. and using a Kayness, Galaxy V capillaryrheometer with a melt temperature of 250° C., melt force of 150 lbs, anda shear rate of 400 sec⁻¹. Parallel plate rheology is measured using aGottfert® 2001 rheometer with a parallel plate radius of 12.5 mm, a gapof 1.0 mm and a melt temperature of 250° C. with a 15% strain. Theresults along with compositional data are set forth below in Table 1.

                  TABLE 1                                                         ______________________________________                                        Example     1A*       1        2      3                                       ______________________________________                                        Composition, pbw                                                              PBT 295.sup.a                                                                             40        40       40     40                                      PBT 195.sup.b                                                                             15        15       15     15                                      Glass fibers.sup.c                                                                        30        30       30     30                                      FRC.sup.d   13.2      13.2     13.2   13.2                                    PTFE.sup.e  1.25      1.25     1.25   1.25                                    Irganox 1076.sup.f                                                                        0.15      0.15     0.15   0.15                                    PE-18.sup.g 0.2       0.2      0.2    0.2                                     DPDP.sup.h  --        0.2      --     --                                      ZnP.sup.i   --        --       0.2    --                                      SAPP.sup.j  --        --       --     0.2                                     Properties                                                                    Viscosity, poise.sup.k                                                        Rheology                                                                      1 minute    5434      4509     4201   5434                                    15 minutes  7647      6649     6325   6375                                    % increase  41        47       51     17                                      30 minutes  14610     7423     7099   12450                                   % increase  169       64       69     129                                     ______________________________________                                         *= Comparative Example                                                        .sup.a = Valox ® 295, General Electric Company, 1100 poise                .sup.b =Valox ® 195, General Electric Company, 300 poise                  .sup.c = OCF 183E, K filament glass, Owens Corning Fiberglass                 .sup.d = Flame retardant concentrate of brominated polycarbonate, antimon     oxide, and a polymer binder                                                   .sup.e = Polytetrafluoroethylene resin dispersion                             .sup.f = Antioxidant, CibaGeigy Company                                       .sup.g = Pentaerythritol tetrastearate                                        .sup.h = Diphenylisodecyl phosphite, GE Specialty Chemical                    .sup.i = Zinc phosphate, Alfa Chemical Company                                .sup.j = Sodium acid pyrophosphate                                            .sup.k = Measured by parallel plate rheology at 250° C.           

A time sweep of the melt viscosity for the above examples is shown inFIG. 2. It can be seen that the compositions stabilized with DPDP, ZnPand SAPP exhibit significantly improved melt viscosity stability overthe control example.

EXAMPLES 4-5

The procedure of Example 1 is generally followed to illustrate theeffectiveness of zinc phosphate as a melt stabilizer in polyesterblends. The results, along with compositional data are set forth inTable 2 below.

                  TABLE 2                                                         ______________________________________                                        Example       4A*    4         5A*  5                                         ______________________________________                                        Composition, pbw                                                              PBT 195.sup.a 50     49.8      50   49.8                                      PBT 295.sup.b 50     49.8      --   --                                        PBT 315.sup.c --     --        50   49.8                                      ZnP.sup.d     --     0.4       --   0.4                                       Properties                                                                    Viscosity, poise                                                              Kayeness.sup.e                                                                T5.sup.f      547    577       1518 1662                                      T10.sup.g     528    544       1303 1439                                      Rheology.sup.h                                                                1 minute      581    610       1589 1868                                      15 minute     756    676       1554 1530                                      % increase    30     10.8      -2   -18                                       30 minute     1109   874       1891 1634                                      % increase    91     43        19   -12.5                                     ______________________________________                                         *= Comparative example                                                        .sup.a = Valox ® 195, General Electric Company, 300 poise                 .sup.b = Valox ® 295, General Electric Company, 1100 poise                .sup.c = Valox ® 315, General Electric Company, 8500 poise                .sup.d = Zinc phosphate, Zn.sub.3 (PO.sub.4).sub.2 · 2H.sub.2 O,     Alpha Chem. Co.                                                               .sup.e = at 250° C., poise                                             .sup.f = dwell time, 5 minutes                                                .sup.g = dwell time, 10 minutes                                               .sup.h = Parallel plate rheology, viscosity vs. time at 250° C.        poise, % viscosity increase from 1 minute                                

It can be seen from the data in Table 2 that zinc phosphate preventsbuild up of melt viscosity in the polyester blends, exhibiting asignificant improvement in maintaining a consistent melt viscosity overtime.

EXAMPLE 6

The procedure of Example 1 is followed to compare the effectiveness ofzinc phosphate stabilizer over other stabilizers. The results, alongwith compositional data are set forth below in Table 3.

                  TABLE 3                                                         ______________________________________                                        Example     6A*       6B*      6C*    6                                       ______________________________________                                        Composition, pbw                                                              PBT 295.sup.a                                                                             40.2      40.2     40.2   40.2                                    PBT 195.sup.b                                                                             15.0      15.0     15.0   15.0                                    Glass Fibers.sup.c                                                                        30.0      30.0     30.0   30.0                                    FRC.sup.d   13.2      13.2     13.2   13.2                                    PTFE.sup.e  1.25      1.25     1.25   1.25                                    Irganox 1076.sup.f                                                                        0.15      0.15     0.15   0.15                                    PE-18.sup.g 0.20      0.20     0.20   0.20                                    Irgafos 168.sup.h                                                                         --        0.20     --     --                                      TPPP.sup.i  --        --       0.20   --                                      ZnP.sup.j   --        --       --     0.20                                    Properties                                                                    Viscosity, poise                                                              Kayenees.sup.k                                                                T5.sup.l    2995      3290     3380   2200                                    T10.sup.m   3311      4144     3411   2149                                    Rheology.sup.n, poise                                                         1 min       5183      4636     4414   4044                                    15 min      10910     7112     7718   5165                                    %.sup.o     110       53.4     75     28                                      30 min      11840     9853     8445   5644                                    %.sup.o     128       112      91     40                                      ______________________________________                                         * = Comparative Example                                                       .sup.a = Valox ® 295, General Electric Company, 1100 poise                .sup.b = Valox ® 195, General Electric Company, 300 poise                 .sup.c = OCF 183E, K filament glass, Owens Corning Fiberglass                 .sup.d = Flame retardant concentrate, PPG Industries                          .sup.e = Polytetrafluoroethylene concentrate                                  .sup.f = Antioxidant, Ciba Geigy Company                                      .sup.g = Pentaerythritol tetrastearate                                        .sup.h = Tris(2,4di-t-butylphenyl)phosphite, Ciba Geigy Co.                   .sup.i = Tetrapotassium pyrophosphate                                         .sup.j = Zinc phosphate, Zn.sub.3 (PO.sub.4).sub.2 · 2H.sub.2 O,     Alpha Chem. Co.                                                               .sup.k = at 250° C., poise                                             .sup.l = dwell time, 5 minutes                                                .sup.m = dwell time, 10 minutes                                               .sup.n = Parallel plate rheology, viscosity vs. time at 250° C.        .sup.o = % viscosity increase from 1 minute                              

Table 3 above clearly demonstrates the improvements in melt viscositystability obtained with the stabilizers of the present invention. Theparallel plate rheology shows a 100% increase in melt viscosity incompositions stabilized with other phosphorus-containing additives,Examples 6A*-6C*, while the composition stabilized according to thepresent invention exhibits less than 50% increase in melt viscosity.Further, the Kayeness data highlights the viscosity increases for thecompositions containing other phosphorus-containing additives.

EXAMPLE 7

The procedure of Example 1 is generally followed to observe the meltviscosity stability of unfilled polyester blends with variousstabilizers. The results, along with compositional data are set forthbelow in Table 4.

                  TABLE 4                                                         ______________________________________                                        Example       7A*         7B*     7                                           ______________________________________                                        Composition, pbw                                                              PBT 295.sup.a 50.0        49.8    49.8                                        PBT 195.sup.b 50.0        49.8    49.8                                        Irgafos ® 168.sup.c                                                                     --          0.4     --                                          ZnP.sup.d     --          --      0.4                                         Properties                                                                    Viscosity, poise                                                              Kayeness.sup.e                                                                T5.sup.f      539         544     577                                         T10.sup.g     513         508     544                                         Rheology.sup.h                                                                1 min         607         590     610                                         15 min        828         812     676                                         %.sup.i       36          38      11                                          30 min        1197        1114    874                                         %.sup.i       97          89      43                                          ______________________________________                                         * = Comparative Example                                                       .sup.a = Valox ® 295, General Electric Company, 1100 poise                .sup.b = Valox ® 195, General Electric Company, 300 poise                 .sup.c = Tris(2,4di-t-butylphenyl)phosphite, Ciba Geigy Co.                   .sup.d = Zinc phosphate, Zn.sub.3 (PO.sub.4).sub.2 · 2H.sub.2 O,     Alpha Chem. Co.                                                               .sup.e = at 250° C., poise                                             .sup.f = dwell time, 5 minutes                                                .sup.g = dwell time, 10 minutes                                               .sup.h = Parallel plate rheology, viscosity vs. time at 250° C.        .sup.i = % viscosity increase from 1 minute                              

The data in Table 4 demonstrates the significant improvements obtainedaccording to the present invention in unfilled PBT blends.

EXAMPLES 8-13

The procedure of Example 1 is generally followed, except employingfurther stabilizers. The results, along with compositional data are setforth below in Table 5.

                                      TABLE 5                                     __________________________________________________________________________    Example   8A* 8   9   10  11  12  13                                          __________________________________________________________________________    Composition, pbw                                                              Blend.sup.a                                                                             100 99.7                                                                              99.4                                                                              99.7                                                                              99.4                                                                              99.7                                                                              99.4                                        ZnP.sup.b --  0.3 0.6 --  --  --  --                                          SAPP.sup.c                                                                              --  --  --  0.3 0.6 --  --                                          SDP.sup.d --  --  --  --  --  0.3 0.6                                         Properties                                                                    Viscosity, poise                                                              Kayeness.sup.e                                                                T5.sup.f  2968                                                                              2267                                                                              2250                                                                              2536                                                                              2631                                                                              2566                                                                              --                                          T10.sup.g 3374                                                                              2276                                                                              2204                                                                              2686                                                                              2691                                                                              2754                                                                              2724                                        Rheology.sup.h                                                                1 min.    5320                                                                              3713                                                                              3725                                                                              4452                                                                              3994                                                                              4421                                                                              3991                                        15 min.   9366                                                                              5281                                                                              4969                                                                              5488                                                                              4828                                                                              4793                                                                              4731                                        %.sup.i   76  42  33  23  21  8.4 19                                          30 min.   12210                                                                             5901                                                                              5664                                                                              8106                                                                              6265                                                                              7809                                                                              5816                                        %.sup.i   130 59  52  82  57  76  46                                          __________________________________________________________________________     *=Comparative Example                                                         .sup.a = PBT 295 (40.2), PBT 195 (15), glass fiber (30), FRC (13.2), PTFE     dispersion (1.25), Irganox ® 1076 (0.15), PE18 (0.2)                      .sup.b = Zinc phosphate, Zn.sub.3 (PO.sub.4).sub.2 · 2H.sub.2 O,     Alpha Chemical Co.                                                            .sup.c = Sodium acid pyrophosphate                                            .sup.d = Monosodium phosphate                                                 .sup.e = at 250° C., poise                                             .sup.f = dwell time, 5 minutes                                                .sup.g = dwell time, 10 minutes                                               .sup.h = Parallel plate rheology, viscosity vs. time at 250° C.        .sup.i = % viscosity increase from 1 minute                              

The data in Table 5 show the improvements in melt viscosity stabilityprovided by compositions of the present invention.

EXAMPLES 14-19

The procedure of Example 1 is generally followed, except employingblends of the low molecular weight PBT with polycarbonate in a non-flameretardant system. The results, along with compositional data are setforth below in Table 6.

                                      TABLE 6                                     __________________________________________________________________________    Example   14A*                                                                              14  15  16  17  18  19                                          __________________________________________________________________________    Composition                                                                   PBT 295.sup.a                                                                           33.45                                                                             33.25                                                                             33.15                                                                             33.25                                                                             33.15                                                                             33.25                                                                             33.15                                       PBT 195.sup.b                                                                           15.0                                                                              15.0                                                                              15.0                                                                              15.0                                                                              15.0                                                                              15.0                                                                              15.0                                        PC.sup.c  20.0                                                                              20.0                                                                              20.0                                                                              20.0                                                                              20.0                                                                              20.0                                                                              20.0                                        SAC.sup.d 1.55                                                                              1.55                                                                              1.55                                                                              1.55                                                                              1.55                                                                              1.55                                                                              1.55                                        ZnP.sup.e --  0.2 0.4 --  --  --  --                                          SAPP.sup.f                                                                              --  --  --  0.2 0.4 --  --                                          DPDP.sup.g                                                                              --  --  --  --  --  0.2 0.4                                         Properties                                                                    Viscosity                                                                     Kayeness.sup.h                                                                T5.sup.i  4670                                                                              3867                                                                              3482                                                                              3287                                                                              3326                                                                              3375                                                                              3457                                        T10.sup.j 6252                                                                              5072                                                                              4190                                                                              3277                                                                              3298                                                                              3311                                                                              3413                                        %         34  31  27  -0.3                                                                              -0.8                                                                              -1.9                                                                              -1.3                                        __________________________________________________________________________     *= Comparative Example                                                        .sup.a = Valox ® 295, General Electric Company, 1100 poise                .sup.b = Valox ® 195, General Electric Company, 300 poise                 .sup.c = Poly(bisphenol A) carbonate, General Electric Company                .sup.d = Mold release/antioxidant stabilizer concentrate                      .sup.e = Zinc phosphate, Zn.sub.3 (PO.sub.4).sub.2 · 2H.sub.2 O,     Alpha Chem. Co.                                                               .sup.f = Sodium acid pyrophosphate                                            .sup.g = diphenylisodecyl phosphite, GE Specialty Chemical                    .sup.h = at 250° C., poise                                             .sup.i = dwell time, 5 minutes                                                .sup.j = dwell time, 10 minutes                                          

The above-mentioned patents and test methods are all hereby incorporatedby reference.

Many variations of the present invention will suggest themselves tothose of ordinary skill in the art in light of the above-detaileddescription. Conventional additives such as clay, mica, pigments andcolorants can be added in conventional amounts. Other phosphitestabilizers such as diisooctyl phosphite, trilauryl phosphite, diphenylphosphite, phenyl diisodecyl phosphite, ethyl hexyl diphenylphosphiteand stearyl phosphite may be employed. Further, the compositions caninclude a wide variety of other thermoplastic resins such aspolycarbonates, polyarylates, polyester carbonates and mixtures thereof;as well as a variety of impact modifers such as core-shell polymers,ethylene vinyl acetate, ethylene ethylacrylate copolymer, SEBS, SBS,EPDM and EPR. All such obvious variations are within the full intendedscope of the appended claims.

I claim:
 1. A thermoplastic resin blend comprising:(a) a relatively lowmolecular weight polybutylene terephthalate resin having a meltviscosity of less than 600 poise; (b) a relatively high molecular weightpolyester resin having a melt viscosity greater than 600 poise; and (c)an effective melt viscosity stabilizing amount of (i) an acid, alkyl,aryl or mixed phosphite having at least one acidic hydrogen or alkylgroup, (ii) a phosphorus oxo acid or (iii) a mixture of any of theforegoing.
 2. A thermoplastic resin blend as defined in claim 1 whereinsaid component (a) has a melt viscosity of less than about 450 poise. 3.A thermoplastic resin blend as defined in claim 1 wherein said component(b) has a melt viscosity of greater than about 900 poise.
 4. Athermoplastic resin blend as defined in claim 1 wherein said component(a) comprises a poly(1,4-butylene terephthalate) resin.
 5. Athermoplastic resin blend as defined in claim 1 wherein said component(b) comprises a poly(1,4-butylene terephthalate) resin.
 6. Athermoplastic resin blend as defined in claim 1 wherein said component(b) comprises polyethylene terephthalate, polycyclohexane dimethyleneterephthalate or a mixture thereof.
 7. A thermoplastic resin blend asdefined in claim 1 wherein said phosphite component (c)(i) comprises aphosphite of the general formula

    P--(OR').sub.3

wherein each R' is the same or different and independently representshydrogen, alkyl, aryl or a mixture of alkyl and aryl provided that atleast one R' group is hydrogen or alkyl.
 8. A thermoplastic resin blendas defined in claim 7 wherein said phosphite component (c)(i) isselected from the group of diisooctyl phosphite, trilauryl phosphite,diphenyl phosphite, phenyl diisodecyl phosphite, ethyl hexyl diphenylphosphite, stearyl phosphite and mixtures of any of the foregoing.
 9. Athermoplastic resin blend as defined in claim 8 wherein said phosphitecomponent (c)(i) comprises diphenylisodecyl phosphite.
 10. Athermoplastic resin blend as defined in claim 1 wherein said phosphorusoxo acid (c) (ii) comprises phosphorous acid, phosphoric acid,polyphosphoric acid, hypophosphorous acid or mixtures thereof.
 11. Athermoplastic resin blend as defined in claim 1 comprising from about 30to about 70 parts by weight component (a), from about 70 to about 30parts by weight component (b), and from about 0.1 to about 10 parts byweight component (c) based on 100 total parts by weight of components(a), (b) and (c).
 12. A thermoplastic resin blend as defined in claim 1further comprising (d) a thermoplastic resin selected frompolycarbonates, polyester carbonates, polyarylates and mixtures of anyof the foregoing.
 13. A thermoplastic resin blend as defined in claim 12wherein said component (d) comprises a poly(bisphenol A)carbonate.
 14. Athermoplastic resin blend as defined in claim 1 further comprising (e) areinforcing agent comprising glass fibers, talc, mica, clay or mixturesthereof.
 15. A thermoplastic resin blend as defined in claim 1 furthercomprising (f) a flame retardant agent.
 16. A thermoplastic resin blendas defined in claim 1 further comprising (g) an additive selected fromimpact modifiers, pigments, fillers, plasticizers, processing aids, UVstabilizers, antioxidants and mixtures of any of the foregoing.
 17. Anarticle prepared from a composition as defined in claim
 1. 18. A processfor stabilizing the melt viscosity of a thermoplastic resin blendcomprising a relatively low molecular weight polybutylene terephthalateresin having a melt viscosity of less than 600 poise; and a relativelyhigh molecular weight polyester having a melt viscosity greater than 600poise, said process comprising adding to said blend an effective amountof a phosphorus compound comprising (i) an acid, alkyl, aryl or mixedphosphite having at least one acidic hydrogen or alkyl group, (ii) aphosphorus oxo acid or (iii) a mixture of any of the foregoing.
 19. Aprocess as defined in claim 18 wherein said relatively low molecularweight PBT resin has a melt viscosity of less than about 450 poise andsaid relatively high molecular weight polyester resin has a meltviscosity of greater than about 900 poise.
 20. A process as defined inclaim 19 wherein said high molecular weight polyester resin comprisespolybutylene terephthalate, polyethylene terephthalate, polycyclohexanedimethylene terepthalate, or a mixture of any of the foregoing.
 21. Aprocess as defined in claim 18 wherein said phosphorus compound isdiphenylisodecyl phosphate.
 22. A thermoplastic resin blend havingimproved melt viscosity stability consisting essentially of(a) a lowmolecular weight polybutylene terephthalate resin having a meltviscosity of less than 600 poise: (b) a high molecular weight polyesterresin having a melt viscosity greater than 600 poise; and (c) aneffective melt viscosity stabilizing amount of (i) an acid, alkyl, arylor mixed phosphite having at least one acidic hydrogen or alkyl group,(ii) a phosphorus oxo acid or (iii) a mixture of any of the foregoing;and, optionally, one or more of the following components: (d) athermoplastic resin selected from polycarbonates, polyester carbonates,polyarylates and mixtures of any of the foregoing; (e) a reinforcingagent; (f) a flame retardant agent; and (g) an additive selected fromimpact modifiers, pigments, fillers, colorants, plasticizers, processingaids, UV stabilizers, antioxidants and mixtures of any of the foregoing.